Server not found in kerberos database

[liulg]

Hi anyone
   I have a problem when i use kerberos.It's version is 5.1.2.3

When i want to get  a ticket by kinit,such as "kinit -f test",Ii can
get a ticket.but when i telnet the telnet server machine
10.30.10.56,through "telnet -a -f -x 10.30.10.56", ,it tell me that
"Server not found in kerberos database".
But when i use "kinit -f test -S telnet/10.30.10.56" or "kinit -f test
-S host/10.30.10.56",it is OK, those exist in /etc/krb5.keytab.
I use "telnet -a -f -x 10.30.10.56", it tell me "telnet: Kerberos V5:
failure on credentials (Matching credential not found)"
why?

my kdc is 10.30.10.56,this machine is not dns.through
"gethostbyaddr",i only get it's ip.OS is RedHat 7.2.
Thanks.